drm/of: make drm_of.h self-contained
Include <linux/err.h> for ERR_PTR.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e9b3ea30984b913f6e32ed35e0575438dff8278c.1709749576.git.jani.nikula@intel.com